In general, an electrically conductive organic polymer called polyaniline is a polymer having a quinonediimine structural unit represented by the formula ##STR2## as the main repeating unit (U.S. Pat. No. 4,615,829 ). Specifically, this polymer is a quinonediimine-phenylenediamine type polyaniline which has a quinonediimine structural unit and a phenylenediamine structural unit both represented by formula (II): ##STR3## as the main repeating units and which is doped with a protonic acid. It is generally said that protonation of nitrogen atoms in the quinonediimine structural units of such a polyaniline is indispensable to development of electrical conductivity. In the above formula, m and n indicate molar fractions of the quinonediimine structural unit and the phenylenediamine structural unit, respectively, in the repeating units, provided that 0&lt;m&lt;1, 0&lt;n&lt;1, and m+n=1.
A film of such an electrically conductive polyaniline can be obtained by dissolving a solvent-soluble quinonediimine-phenylenediamine type polyaniline in a proper solvent, casting the solution followed by drying to form a film, and then subjecting the film to a doping treatment (protonic acid doping) by immersing the film in an aqueous solution of a protonic acid, as described in, e.g., JP-A-3-28229. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, long time has usually been required to dope with a protonic acid after the preparation of a solvent-soluble quinonediimine-phenylenediamine type polyaniline film as described above. Illustratively stated, although doping time varies considerably depending on the kind of the protonic acid used and the properties of the polyaniline film, such as film thickness, porosity, etc., the doping of a polyaniline film having a thickness of about 20 .mu.m, for example, necessitates several-hour immersion in an aqueous hydrochloric acid solution in order to impart an electrical conductivity of 1 S/cm or higher. When p-toluenesulfonic acid is used as the acid, several days may be required. In the case of using an acid having a high molecular weight such as 1,5-naphthalenedisulfonic acid, more prolonged time may be required.
In another method, p-toluenesulfonic acid is dissolved in a solution of a quinonediimine-phenylenediamine type polyaniline of the above described kind to prepare a doped-state polyaniline solution, and this solution is cast and dried to thereby obtain an electrically conductive film. This method, however, is disadvantageous in that only a limited number of protonic acids can be used to obtain a quinonediimine-phenylenediamine type polyaniline which is in the doped state and is solvent-soluble, and that a polyaniline generally has a far lower solubility in the doped state than in the undoped state and, in particular, the solubility is significantly decreased by a polyvalent acid such as 1,5-naphthalenedisulfonic acid. Instability of the solution is also disadvantageous in industrially utilizing the solution.
JP-A-3-52929 discloses a process for producing an electrically conductive polyaniline film which comprises dissolving in a solvent an imino-p-phenylene type polyaniline having an imino-p-phenylene structural unit represented by formula (I): ##STR4## as the main repeating unit, adding a proper oxidizing agent to the resulting solution, and then casting the mixture to form the film. According to this process, the imino-p-phenylene type polyaniline is converted by oxidation into a quinonediimine-phenylenediamine type polyaniline. In this case also, the polyaniline is doped with anions of the oxidizing agent. Since this doping is accompanied by an oxidation reaction, it is usually called oxidation doping.
However, such a polyaniline solution also has a problem concerning solution stability because the polyaniline has been converted to a quinonediimine-phenylenediamine type polyaniline in the doped state.
For improving the solution stability, a method has been proposed which comprises adding a reducing agent such as phenylhydrazine to a solution of a quinonediimine-phenylenediamine type polyaniline to convert the polyaniline to the imino-p-phenylene type thereby improving the solubility, and then adding a dopant to the solution. However, since the imino-p-phenylene type polyaniline solution containing a dopant gives a film in which the polyaniline is based on the imino-p-phenylene structure and hence contains the above-described quinonediimine structure in a small proportion, it is necessary that, for development of electrical conductivity, the imino-p-phenylene structure be oxidized into the quinonediimine structure. This oxidation reaction may require about one week when, for example, the film has a thickness of about 20 .mu.m and the oxygen in air is used for the oxidation. The necessity of such a long time for the development of electrical conductivity is exceedingly disadvantageous industrially.
As a method for overcoming the above problem, a method of obtaining an electrically conductive thin film is proposed in JP-A-3-285983 which method comprises dissolving an ammonium salt of a protonic acid in a quinonediimine-phenylenediamine type polyaniline solution, applying this solution on a suitable substrate, and then drying the coating to thereby evaporate and remove the highly volatile ammonia or organic amine while remaining the less volatile protonic acid in the thin polyaniline film to make the polyaniline in the doped state.
As a result of further investigations by the present inventors on the above method for the practical application of the method, it was found that the organic amine does not always evaporate readily during drying when the film thickness is 1 .mu.m or more, particularly 10 .mu.m or more, and hence that there are cases that it is difficult to obtain sufficient electrical conductivity.
On the other hand, JP-A-60-133027 and Faraday Discuss. Chem. Soc., 88, 317 (1989) disclose that a quinonediimine-phenylenediamine type polyaniline can be obtained by subjecting a powder of an imino-p-phenylene type polyaniline to an oxidation doping treatment with an acetonitrile solution of ferric chloride or with hydrogen peroxide and a solution of an inorganic acid such as hydrochloric acid, sulfuric acid, or phosphoric acid.
The present inventors also have found that the oxidation doping of an imino-p-phenylene type polyaniline film with a thickness of, e.g., about 20 .mu.m using p-quinone or hydrogen peroxide as the oxidizing agent and using any of various organic acids as the protonic acid proceeds far more speedily than the above-described protonic acid doping treatment.
Although such oxidation doping is more practical than the protonic acid doping treatment, it still requires much time if a protonic acid having a relatively large molecular size such as 1,5-naphthalenedisulfonic acid is used as the dopant. For example, in the case of using a water/ethanol mixed solvent containing both p-quinone and 1,5-naphthalenedisulfonic acid, the attainable electrical conductivity is as low as 10.sup.-1 S/cm or less even when the treatment is conducted for 60 minutes.